Methods for monitoring age-related changes in transformer oils

This paper deals with four most established physical and chemical methods for monitoring age-related changes in transformer oils. It presents the findings of acidity, interfacial tension, dielectric dissipation factor, and IR spectrophotometry tests, carried out on six samples of new mineral transformer oils from various suppliers, after artificial oil aging according to the IEC 61125 C standard method. Also test results from twelve differently aged operating transformers oil samples are presented. A comparative sensitivity analysis was carried out for each test method. The findings were considered from the aspect of the degree of sensitivity for measuring oil aging processes degree in the individual methods, which is significant for the appropriate assessment of operating transformer oil suitability and issuing timely recommendations for implementing maintenance measures on a transformer. The four test methods discussed were also considered in terms of complexity, oil sample size and test measurement uncertainty. The paper shows the valid criteria for transformer oils quality according to IEC 60296 and IEC 60422 standards and critically assesses methods for establishing the aging degree of oils. A proposal for improvement and revision of the IEC 61125 C standard has been presented that takes into account the use of various methods for assessing the oxidation stability of new mineral oils.